SUMMARY: PRETREATMENTS BY MATERIAL

These, then, are the major sample materials submitted to radiocarbon laboratories and the methods most often used in pretreating and extracting fractions for dating.

(a). Charcoal: In some cases, charcoal should be identified prior to radiocarbon dating and only short-lived species selected for dating. Any surface contamination is removed. The sample is crushed and physical pretreatment undertaken to remove rootlet contaminants. Acid washing is undertaken to remove non-sample carbonates. An alkali treatment is used with samples suspected of soil humic contamination. This is followed with an acid wash to ensure that atmospheric CO2, absorbed during the base wash is removed. Solvent extractions may be undertaken if petrochemical contamination has occurred or if resins need to be removed.

(b). Lake Muds: (gyttja). The sample is slurried and wet seived to create two fractions, coarse and fine. The coarse fraction is stored as a reference. The fine fraction is acid washed and may also have an alkali pretreatment depending on the characteristics of the environment from which it originated.

(c). Peat: The same pretreatment applies to peat as to lake muds.

(d). Soil: Same as above.

(e). Shell: The shell species which comprise the sample will need to be identified. Lacustrine and riverine shell may involve considerable errors unless some background research has been undertaken regarding the reservoir of the shell. In samples which may have recrystallised, an XRD analysis should be considered, or examination under high powered, or SEM. Shell is cleaned in an ultrasonic bath, shell exteriors are scraped with a dental drill or acid washed using dilute HCl and rinsed and dried. The samples are crushed prior to hydrolysis and dating.

(f). Wood: Exterior removed using hammer and chisel, solvent extraction carried out using soxhlet extractor, to remove waxes and resins. Starch fraction removed using water and soxhlet extractor. HCl and NaCLO2 used to isolate cellulose fraction, H2SO4 used to obtain lignin. Alpha-cellulose obtained by washing in hot dilute NaOH and then acid washing using dilute HCl, rinsed and dried.

(g). Bone:

There are five basic steps in the pretreatment of bone samples for conventional dating methods:

1. Cleaning
Cleaning includes removal of roots, preservatives and if plenty of sample exists, removal of the most contaminated bone. This may be done by blasting, scraping and/or washing in an ultrasonic bath, while preservatives may be treated with methanol at 60'C, or acetone at room temperature and evaporated to dryness.

2. Grinding
Sizes range from 0.5mm to 5mm. The smaller fractions are, however, generally used so that a shorter demineralization time is needed.

3. Decalcification:
Most common used decalcification techniques are HCl and EDTA treatments. These treatments remove apatite (carbonates and phosphates) but leave humic contaminants as solid phases. The HCl treatment is generally carried out at low temperature (4-6'C) and low concentrations (around 0.5N), until pH is stable. The low temperature and acid concentration minimises hydrolysis of the collagen molecule. The acid insoluble component is then filtered or centrifuged and rinsed with distilled water until neutral.

4. Alkali treatment (optional).
Treatment with NaOH removes base-soluble contaminants such as humic acids and some lipids. Generally the acid insoluble fraction is briefly treated at room temperature with low (0.1N NaOH), though if sample quantity is not a problem, longer time may apply. Unfortunately collagen solubility also increases when NaOH concentrations are greater than 0.1 NaOH (Arslanov et al. 1993:389), as a consequence the success of NaOH is debatable. Some consider its use to be no more effective than the gelatinization step (Redvers-Newton and Coote 1994:273), or too destructive (Stafford et al. 1988:2264-5). Gurfinkel (1987:51), on the other hand, concluded that alkali extraction was more effective at removing humic contaminants than either the HCl or gelatinization methods. The residue is then rinsed to neutrality.

5. Gelatinization (optional).
The most insoluble "collagen" residue is denatured in acidic hot water. This largely leaves humic acids as residues which may be subsequently filtered off. The pH of the water is generally agreed to be most affective at around 3, but temperature and length of time vary considerable between labs (ranges between around 70'C to 120'C for 1/2 to 18 hours). The whole gelatinization procedure may be done in a sealed nitrogen atmosphere in order to prevent caramelization of the carbohydrates in the gelatin (Stafford et al. 1987; Redvers-Newton and Coote 1994). The supernatant is then centrifuged or filtered, and air dried or freeze dried.

Pretreatment may terminate at any step after decalcification depending on quantity and quality of extracted "collagen", and can be followed with column chromatography purification or ultrafiltration techniques (see Brown et al. 1988; Stafford et al. 1987, 1988; Law and Hedges 1989; Long et al. 1989; van Klinken and Mook 1990:155; Law et al. 1991; van Klinken and Hedges 1992). The final composition of the insoluble residue will depend on the decalcifying conditions (i.e. length of decalcification, temperature, strength of acid)(Hedges and van Klinken 1992:282; Arslanov et al. 1993:389).